The field of the present invention is bearing systems for turbo machinery.
Turboexpanders typically include a radial inflow turbine rotor mounted within a housing having a radial inlet and an axial outlet. The turbine rotor is rotatably mounted within bearings through a shaft fixed to the rotor. Such turboexpanders may be used with a wide variety of different gas streams for such things as air separation, natural gas processing and transmission, recovery of pressure letdown energy from an expansion process, or thermal energy recovery from the waste heat of associated processes.
For air separation applications, turboexpanders are designed to optimize the insulation of the cold sections of the expander to realize high efficiencies at design conditions. In high-pressure applications, these machines are capable of expansion beyond the critical point. In low-pressure and medium-pressure applications, energy from the turboexpander can be used as the final stage of the recycle compressor. Compressors can be associated with such turboexpanders as a means to derive work or simply dissipate energy from the turboexpander.
In turboexpanders and other such fluid processing turbo machinery, the differential pressure across the rotor of the device can lead to thrust loads on the associated shaft. Depending on the pressure gradient and the configuration of the rotor, an axial thrust load will develop that will vary depending on such things as differential pressure between input and output and rotor speed. Two mechanisms for adjusting or accommodating thrust loading are illustrated in U.S. Pat. No. 3,895,689, issued Jul. 22, 1975, for Thrust Bearing Lubricant Measurement And Balance, and U.S. Pat. No. 4,385,768, issued May 31, 1983, for Shaft Mounting Device And Method, the disclosures of which are incorporated herein by reference. In both devices, axial position of the shaft is sensed. A control passage extends from a low-pressure position on the device to a high-pressure position. The passage is controlled by a valve which is responsive to the shaft position as detected by the sensor. By selectively venting the high pressure on the back of the rotor, thrust loading resulting from the differential pressure across the rotor can be modified. In both examples, thrust bearings are employed in conjunction with the thrust control system. Conventional bearings are shown.
Two primary types of bearings that may be used to support the rotor shaft in turbo machinery are magnetic bearings and oil film bearings. Magnetic bearings provide superior performance over oil film bearings. Magnetic bearings have low drag losses, high stiffness and damping, and moderate load capacity. In addition, unlike oil film bearings, magnetic bearings do not require lubrication, thus eliminating oil, valves, pumps, filters, coolers and the like with the risk of process contamination.
There are two types of magnetic bearings, active and passive. Passive magnetic bearings inherently lack damping properties and, as such, are not suitable for application in dynamic industrial equipment such as compressors, turbines, pumps, motors, and other rotating equipment.
Active magnetic bearings are characterized by a ferromagnetic rotor shaft that is surrounded by electromagnetic coils and by position sensing and control electronics. The shaft assembly is supported by active magnetic radial bearings at appropriate positions on the rotor shaft. The magnetic radial bearings levitate the rotor shaft using a continuously-controlled magnetic field. Centralized active magnetic thrust bearings are typically used to control the axial position of the rotor shaft. An active magnetic thrust bearing system typically includes opposed electromagnetic coils fixed to a housing with thrust disks positioned on the rotor shaft. One bearing or the other may be activated by a controller to repulse a disk as a mechanism for axially recentering a shaft.
Antifriction bearings as well as seals may be installed at each end of the rotor shaft to support it when the magnetic bearings are not energized. This avoids any contact between the rotor shaft and the stator of the radial magnetic bearings. These auxiliary or "back-up" bearings are dry lubricated, and they remain unloaded during normal operation.
The conventional method of detecting the axial position of the rotor shaft in order to compensate for axial thrust variations employs noncontacting probes. Each probe produces an output voltage that is proportional to the detected size of the gap between the probe and a discontinuity on the shaft such as a collar. Using proprietary control loops, active magnetic bearings adjust thrust bearing magnetic fields in accordance with the probe output voltages to maintain a balanced axial position of the rotor shaft. When the axial position of the rotor shaft changes, the detected size of the air gap changes, and the probes, in response, alter their output voltages.
Electronic control systems for active magnetic bearings control the position of the rotor shaft by adjusting current to the electromagnets in response to signals from the shaft position sensors or probes. When the electronics energize the electromagnetic coils, attractive forces between the rotor shaft and the coils cause the rotor shaft to be suspended and axially positioned. Sensor readings are combined using control loops that are proprietary to the bearing manufacturers, enabling active magnetic bearings to adjust thrust bearing magnetic fields in accordance with the probe output voltages. This maintains a balanced axial position of the rotor shaft without regard for elliptical or triangular deformation of the rotor shaft surface. The sensor readings may thus be used to automatically cancel rotation signal harmonics.
The electronics may also be used to adjust the stiffness and damping of the active magnetic bearings for specific roto-dynamic applications. The electronic control provides more effective damping than is possible with mechanical arrangements. Electronic adaptation to stiffness in the control system allows the first bending critical speeds to be located safely out of the nominal operating range. A high damping factor can be established to allow the rotor shaft to safely pass across critical speeds.
During operation, high-speed turbo machinery is subject to process pressures and flow variations. Many of the maintenance problems experienced with turbo machinery originate in the thrust bearings. This is primarily the result of wide variations in thrust loading. Process-induced variations in axial thrust can cause thrust overloads in turboexpanders in general and in compressor-loaded turboexpanders in particular. Off-design operation and unforeseen transient process conditions can also trigger overloads. These overloads may damage internal parts of the turbomachinery and, in some instances, may cause major failure. Active magnetic bearings that comprise radial and double-acting thrust bearings monitor the axial position of the rotor shaft using position sensors or probes. If a thrust-force imbalance occurs, magnetic force acting on one of the two thrust bearings is adjusted to maintain the rotor shaft in its axially-balanced position. Such active magnetic thrust bearings, such as those manufactured by S2M (Societe de Mecanique Magnetique) of France, provide an advantageous alternative to conventional oil bearing systems for many turbo machinery applications.
A number of advantages are associated with magnetic bearings. By suspending the shaft in magnetic fields, there are virtually no friction surfaces to wear and require frequent attention. No lubrication is required with its inherent danger of process contamination and its required lubrication support equipment. Very high rotational speeds can also be achieved because of the elimination of friction surfaces. With minimal friction, energy losses are minimized. A wide variety of temperatures may also be used as the bearings do not employ lubricants affected by temperature. The available electronics for controlling such bearings make possible accurate control, minute system changes, adjustable stiffness and damping, and reliable control signals that may be employed for automatic shutdown. The inherent nature of such bearings also allows the shaft to rotate about an inertial axis rather than a geometric axis to substantially reduce imbalance and equipment vibration problems.
Despite the advantages of using active magnetic bearings in turbomachinery, damaging thrust overloads remain a possibility. For example, off-design operation or unforeseen transient process conditions can trigger overloads. Axial rotor shaft position control is typically not capable of compensating for, or reacting quickly enough to, excessive axial thrust variations. The thrust load capacity of the thrust bearings in such active magnetic bearings is understandably physically and mechanically limited. To protect turbo machinery from excessive thrust forces, these active magnetic bearings are equipped with an alarm to shut the equipment down when the thrust load becomes excessive.